Fears over radiation from the tsunami-stricken Fukushima nuclear plant and fall-out from the bloody civil war in Syria are hanging over Tokyo and Istanbul's attempts to host the 2020 Olympics, amid suggestions that third candidate city Madrid could benefit.

Tokyo

Pros

Hosted successful Games in 1964

Stable political climate

Offers unmatched personal safety and financial security

World-class infrastructure for a compact Games, with 85 percent of the Olympic venues located within eight kilometres of athletes' village

Organisers say strict anti-quake measures mean the city is safe

Cons

Fears over Fukushima's radioactive fallout

Areas of the country still not entirely recovered from 2011 tsunami

Tokyo's bid leaders were forced to assuage lingering doubts over safety in the Japanese capital, with radioactive water leaking into the ocean, two-and-a-half years after the nuclear disaster.

Bid president Tsunekazu Takeda even revealed that he had written to every member of the International Olympic Committee (IOC), to try to allay fears.

"The water is safe and the level of radioactivity is absolutely safe," he told a news conference in Buenos Aires, where IOC members vote on Saturday to decide the 2020 host city.

"Our prime minister (Shinzo) Abe has officially announced that the government will be responsible for the project (to clear up Fukushima).

"I am not worried about the Tokyo 2020 bid."

But the 2011 meltdown at the plant 220 kilometres from Tokyo, which followed a devastating earthquake and tsunami that killed more than 18,000, could still be a deciding factor in the vote, according to one analyst.

Syria conflict affecting Istanbul bid

Istanbul

Pros

First predominantly Muslim nation to host the Games

Unique geographical location offers the first Olympics to take place on two continents, Europe and Asia

Cons

Close proximity to Syrian bloodshed

Lingering anti-government sentiment after last year's protests

Spread out Olympic venues with a transport system that is a daily nightmare for the inhabitants

Majority of Olympic sites are yet to be built, notably the stadium which is due to host both the Opening and Closing Ceremonies

Of the major projects, only the Ataturk Stadium already exists

Wolfgang Maennig, a sports economics specialist from Hamburg University, said the brutal civil conflict in Turkey's neighbour Syria, which has seen more than two million people flee the country, according to the United Nations, could also sway members away from choosing Istanbul.

"To my astonishment, it seems like it's going to be Madrid," he was quoted as saying by the Huffington Post on Wednesday evening.

"I talked today to the president of a national (Olympic) federation and people are still afraid of Syria and even Iraq, even though that was years ago, affecting Turkey.

"There are also many concerns about the level of radiation in Japan."

However, Singaporean IOC vice-president Ng Ser Miang, a candidate to succeed outgoing president Jacques Rogge, said he had not sensed fellow IOC members getting nervous over the situation.

"Current events do play a role but the world is an ever changing place and who knows what it will be like in seven years time, it is a vote for the future not just about the present," he said.

Maennig, a former Olympic rower who won gold for West Germany in the men's coxed eights in Seoul in 1988, added that he favoured Istanbul but they "lost their credibility with the protests" sparked on May 31 by an environmental battle to save a park from redevelopment.

The protests - and the heavy-handed crackdown that followed - escalated into mass displays of anger against Prime Minister Recep Tayyip Erdogan, tarnishing Turkey's image abroad.

Istanbul is also facing a slew of positive doping tests among Turkish athletes, although the country's sports administrators maintain that the high number of cases, 31, is down to better and more comprehensive testing.

Voting leaks may cost Madrid

Madrid

Pros

Politically stable

Cultural hub

Compact Games - most venues within 10km of city centre

The city already boasts well-developed public transport links

80 per cent of required infrastructure already built

Cons

Voting numbers were leaked to Spanish newspaper

Economic trouble since GFC

Intensive lobbying is expected in the run-up to Sunday (AEST) morning's vote but Madrid, which hopes that a successful bid could kick-start a recession-hit economy, saw momentum briefly stopped by an apparent leak of IOC voting intentions.

Spanish paper El Mundo published names - some with photos - of up to 50 IOC members who were going to vote for them.

While 50 votes would give Madrid an unlikely win in the first round of voting, the revelations did not sit well with the members, who vote in a secret ballot.

History suggests it could spell trouble as France's Le Parisien newspaper did a similar thing before the vote on the 2012 Games hosts in 2005, which saw London edge out Paris in a shock result.

One senior IOC member said the revelations in El Mundo had gone down "like a lead balloon" with the members.

But outgoing IOC President Jacques Rogge, whose successor will be appointed next Tuesday, said he did not believe that Madrid would suffer from the revelations.

"I would say that one shouldn't pay any attention or give credit to this type of information," said the 71-year-old, who has been at the helm for the last 12 years.

"Only the person who presses the button (the way the IOC members vote) on Saturday knows how they are going to vote.